Measuring the speeds of rotation of the vehicle wheels for control of the braking force, drive force and/or driving dynamics of a motor vehicle in open or closed loop is known. To do this in conventional manner, various methods (e.g. Hall or magneto-resistive sensors) are used. In addition, measuring the wear of the brake pad of a vehicle is known in that, for example, contact pins are embedded at a specific depth of the brake pads, which trigger a contact upon actuation of the brake when the brake pad is worn to this depth.
For example, the article “Integrierte Hall-Effekt-Sensoren zur Positions-und Drehzahlerkennung” (Integrated Hall Effect Sensors for Position and Speed Recognition) of the journal “elektronik industries,” 7-1995, pp. 29-31, describes active sensors for use in the motor vehicle for anti-lock braking, traction control, engine and transmission open-loop and closed-loop control systems. Such sensors supply two current levels in a two-wire circuit which are converted into two voltage levels by a measuring resistor in an appropriate controller.
In addition to the Hall effect sensors, the use of magneto-resistive sensors is also known for speed recording, e.g., from the article “Neue, alternative Lösungen für Drehzahlsensoren im Kraftfahrzeug auf magnetoresisitiver Basis,” (New Alternative Solutions for Speed Sensors based on Magnetoresistance of the Motor Vehicle), VDI Reports No. 509, 1984.
German Patent No. 26 06 012 (U.S. Pat. No. 4,076,330) describes a special common arrangement for detecting the wear on a brake pad and for detecting the wheel speed. To do this, the brake pad wear detected and the wheel speed detected by an inductively operating sensor are supplied via a common signal line to an analyzer. This is achieved in that the wheel speed sensor is completely or partially short-circuited in response to a detected brake pad wear.
Other systems as described, for example, in German Patent No. 43 22 440, require at least two signal lines between one wheel unit and the analyzer for detecting the speed and the brake pad wear on a wheel and a wheel brake, respectively.
In the speed detection method mentioned above, it is known that the air gap between the rotating ring gear and the actual sensor element has a considerable influence on the quality of the speed signal. Reference is made in this respect to e.g., German Patent Application No. 32 01 811.
The above-mentioned information (for example, brake pad wear and air gap/signal quality) is generally detected near the wheel and evaluated in a control unit mounted at a distance from the wheel. To do this, the information must be transmitted to the controller.
German Patent Application No. 196 18867.9 describes how to modify a rotational-speed signal in various specifiable ways for transmitting additional information (excessive brake pad wear, air gap that is too large/defective signal quality). The modification is carried out in different ways depending on the additional information to be transmitted. Making the different modifications of the speed signal requires a certain amount of effort.
The object of the present invention is to implement a very simple and reliable transmission of the speed signal and other information.